1. Field of the Invention
The present invention is directed to a method and system for measuring and controlling the thickness of coatings or films deposited on the walls of and surfaces within a plasma processing chamber during etch and/or deposition processes.
2. Discussion of the Background
Depending on the particular processes for which a plasma processor has previously been used, the composition and morphology of coatings or films on the processor walls can vary. For example, in Plasma Enhanced Chemical Vapor Deposition (PECVD) processing, the deposition material is, in fact, deposited not only on the semiconductor wafer being processed, but also on virtually every exposed surface within the reactor.
Similarly, during plasma etching, some of the material removed from the wafers will coat virtually all exposed surfaces of the reactor, rather than be removed from the reactor by the vacuum pump. For example, in the oxide plasma photoresist etch process, the walls acquire a coating of organic material that includes both photoresist redeposited by the plasma and plasma-polymerized etch gas. The removal of such deposits by ion bombardment may involve sputtering or some form of a chemically assisted process such as low pressure ashing. These coatings may affect the performance of the plasma processor in different ways. In particular, if the thickness of a coating becomes sufficiently great, coating fragments may dislodge and cause particulate contamination of the plasma processor. Similarly, the surfaces of semiconductor wafers are detrimentally contaminated as well.
Another problem may arise if the coating is subject to sputtering or evaporation during a plasma process. In such cases, molecules, atoms, ions, etc. of the coating may alter the plasma chemistry and, thereby, affect the plasma process. Furthermore, it is also possible for such constituents of the plasma to be incorporated in a film being deposited on a wafer, with potentially undesirable consequences.
Techniques have been developed for removing coatings or films deposited on the walls of an inductively coupled plasma reactor. For example, Blalock (U.S. Pat. Nos. 5,514,246 and 5,647,913) describes an apparatus for cleaning the interior surfaces of the walls of an inductively coupled plasma reactor. In Blalock, the reaction chamber is bounded by a dome-shaped shell constructed from an insulating material (e.g. alumina or quartz) and an electrically conducting, grounded base plate. Exterior to the dome-shaped shell there is a helical coil that is excited by an Radio Frequency (RF) signal. A metallic (i.e., electrically conducting) electrostatic shield is placed between the outer wall of the insulating reactor and the helical excitation coil. This electrostatic shield prevents any capacitive coupling between the RF induction coil and the plasma. The metallic shield typically has a number of slots directed in such a way as to substantially eliminate eddy currents that would, in the absence of the slots, be excited in the metallic shield by the RF current in the induction coil. In Blalock, a switch connects the electrostatic shield either to an RF ground or to an RF signal source, depending on the particular intended procedure. This configuration reduces the effectiveness of the RF ground.
In a typical deposition or etch process, gases appropriate for the intended purpose are introduced into the reaction chamber and the plasma is excited by means of a nominally sinusoidal RF current that flows in the induction coil. The frequency of the exciting RF current is typically in the range from a few MHZ to 100+MHZ. A common frequency is 13.56 MHZ. In practice, however, the exciting RF current may include significant harmonic components due to the complicated interactions between the plasma and the RF source.
For a cleaning procedure, Blalock teaches that the ground connection to the electrostatic shield is to be removed, the shield is to be connected to an RF signal source, and gases appropriate for the removal of the films previously deposited on the surfaces inside the reaction chamber are to be introduced into the reaction chamber. The plasma is then excited by capacitive means, using the electrostatic shield as one electrode and one or more conductors inside the reaction chamber as the other electrode(s). The interior surfaces of the reaction chamber are subjected to the plasma until the unwanted, previously deposited films have been removed.
It is one object of the present invention to measure film/coating growth in a plasma processing system.
It is another object of the present invention to control film/coating growth in a plasma processing system.
It is yet another object of the present invention to provide a better RF ground than known prior art systems.
These and other objects of the present invention are provided in part by introducing a second conducting shield, referred to hereinafter as a xe2x80x9cbias shield,xe2x80x9d between the exterior reaction chamber wall and the electrostatic shield. Moreover, the present invention monitors various constituents in the vacuum system exhaust gases during cleaning via one or both of two techniques. The first technique uses microwave transmission and reflectometry, and the second technique uses optical interference and reflectometry.